The present invention relates to a print inspection method, a print inspection apparatus, and an automatic print sorting system, and more particularly to a method of and an apparatus for inspecting whether a print pattern composed of dots printed by an ink jet printer or similar printer is acceptable, and an automatic system for sorting acceptable print patterns.
In the production of foods, beverages, and other products, it has been customary to indicate product type, date of production, and other information on the products as a series of letters, numerals, and symbols to distinguish them. These letters, numerals, and symbols are usually printed on the surface of a product or on a label or similar object attached to the product by an automatic printing apparatus. At times, there may be caused print failures such as missing letters or numerals, blurring, erasuring, double printing, ink stains, and skewing. Since the printed letter, numerals, and symbols on products represent use, performance, duration of use, product liability, and other data, it is necessary to inspect all of the printed products to see whether the printed patterns are acceptable.
The inventor studied the inspection of printed patterns on a canned beer production line prior to the present invention.
On a canned beer production line, a large number of canned products are moved by a conveyor at such a high speed that the time interval between two adjacent products as they pass through a certain fixed point is, for example, one-tenth second or less. Such a high-speed production line employs an ink jet printer for printing a desired print pattern on rapidly moving products by applying ink particles from an ink jet nozzle to the products at a high speed.
It is impossible to rely upon a visual inspection process for checking whether the printed patterns are acceptable or not. At present time, the printed information is checked by sampling the products as they are printed by the ink jet printer.
There has been a strong demand for the automatic inspection of printed patterns on a high-speed production line with two adjacent products spaced by, for example, more than one-tenth second.
The inventor has analyzed the automatization of inspection of printed patterns on such a high-speed production line, and, as a result, found that an existing inspection system based on image recognition technology cannot be applied directly to automatic print inspection because of various problems described below.
Since the printed products move rapidly, the printed patterns must be inspected quickly.
A letter, a numeral, or a symbol printed by an ink jet printer comprises a matrix of dots. A line segment of a printed letter, for example, is composed of closely spaced dots. Because a printed pattern is represented by a cluster of ink dots, machine-based automatic recognition requires a determination of where the cluster of dots is terminated, i.e., a determination of the spatial extent of one group of related dots. Automatic recognition of a character composed of dots is difficult to achieve, unlike automatic recognition of a character made up of continuous line segments.
As described above, a pattern is printed on a product by an ink jet printer by applying ink at a high speed to the product which is rapidly moving. Therefore, the printed letter, numeral, or symbol is often subjected to deformation such as elongation, contraction, or distortion. Frequently, printed dots may be positioned in contact with each other or too widely spaced from each other. Difficulty arises in establishing a criterion for determining whether a certain printed pattern is acceptable.
According to one conventional print inspection method, a standard pattern composed of continuous line segments is registered in advance, and a printed pattern is inspected by comparison with the registered standard pattern to see if the inspected pattern is in accord with the standard pattern. The inventor's research indicates that, if such an existing print inspection method were employed to inspect a pattern printed by an ink jet printer, most of inspected patterns would not match the standard pattern and would be rejected as unacceptable as a result of pattern matching. Lowering the criterion would solve the problem, but would be liable to judge those printed patterns which do not meet a desired standard as acceptable, and hence would fail to carry out an appropriate inspection process.
Dates of production printed on beer cans change as needed. Therefore, in the print inspection method employing a registered standard pattern for print inspection, it is necessary to register a standard pattern as often as such dates change. The registration of standard patterns is a special process different from the inspection process itself on the production line. The registration process is complex and puts an extra burden on the operator.